The present invention relates to an image display device and a method of driving the same, particularly to a matrix-type liquid crystal display device performing a multiplex driving.
In an image display device represented by liquid crystal display elements, when the number of segments or the number of pixels is large, a multiplex driving of a time division driving system employing a matrix electrode, is performed. In the structure of the matrix electrode, a pair of electrode substrates are opposingly arranged, a plurality of strip-like row electrodes (X-electrode) are parallely arranged on a first substrate, a plurality of strip-like column electrodes (Y-electrode) are parallely arranged on an opposing second substrate, which are orthogonal to the row electrodes, and a liquid crystal is enclosed and interposed between the both electrode substrates.
In the multiplex driving in such a matrix-type liquid crystal display device, a signal of a row electrode waveform composed of a selecting voltage and a non-selecting voltage is applied on the row electrode in a predetermined frame period, in synchronism therewith, a signal of a column electrode waveform composed of an ON-voltage and an OFF-voltage is supplied on the column electrode and a successive line scanning is performed, thereby performing the display by exciting voltages at liquid crystals at desired matrix intersection point positions (pixel position).
As a method of driving a simple matrix-type liquid crystal display device, a method is known wherein voltages at selected points and unselected points on the matrix, are averaged thereby reducing an influence of the "cross effect" as little as possible. The driving waveforms are shown in FIGS. 8A through 8C and FIGS. 9A through 9C. FIG. 7 shows a display state of a liquid crystal panel to be displayed by these driving waveforms. In FIG. 7, a liquid crystal panel having a 7.times.7 dots construction, is shown. However, the number of dots in an actual liquid crystal panel is far more larger than that in FIG. 7. The display dot in a hatched portion indicates an ON-state (switch on state), whereas the display dot at a white portion, an OFF-state (switch off state).
In the respective row electrodes C1 through C7, only a single row electrode is selected by successively applying the selecting voltage, and the non-selecting voltage is applied thereon in an unselected time. Furthermore, simultaneously, the ON-voltage or the OFF-voltage is applied on the respective column electrodes S1 through S7. That is to say, when a dot at an intersection point of a certain row electrode and a certain column electrode, is to be switched on, the ON-voltage is applied on the column electrode when the row electrode is in a selected state, whereas, when it is not to be switched on, the OFF-voltage is applied thereon when the row electrode is in a selected state.
Examples of actual driving waveforms are shown in FIGS. 8A through 8C and FIGS. 9A through 9C. FIG. 8A shows a driving waveform applied on the row electrode C1, FIG. 8B, a driving waveform applied on the column electrode S2, and FIG. 8C, a driving waveform applied on a dot at the intersection point of the row electrode C1 and the column electrode S2. FIG. 9A shows a driving waveform applied on the row electrode C2, FIG. 9B, a driving waveform applied on the column electrode S5, and FIG. 9C, a driving waveform applied on a dot at the intersection point of the row electrode C2 and the column electrode S5.
In FIGS. 8A through 8C and 9A through 9C, F1 and F2 designate frame periods. During the frame period F1, V5 designates a selecting voltage, V1, a non-selecting voltage, V0, an ON-voltage and V2, an OFF-voltage. During the frame period F2, V0 designates a selecting voltage, V4, a non-selecting voltage, V5, an ON-voltage and V3, an OFF-voltage. In these Figures, V5-V4=V4-V3=V2-V1=V1-V0=V and V5-V0=bV where b is a bias value. In this way, an alternating current driving is performed by changing the polarity of signal during the frame periods of F1 and F2.
As is known by the comparison between FIGS. 8A through 8C and FIGS. 9A through 9C, whether the dot to be displayed is in the ON-state or in the OFF-state, is determined by whether the ON-voltage is applied on the column electrode or the OFF-voltage is applied thereon, when the row electrode including the dot to be displayed is applied with the selecting voltage.
This driving method is called Optimized Amplitude Selective addressing method which has conventionally been performed.
FIGS. 10A and 10B shows a conventional example of a method of supplying the respective voltages of V0, V1, V2, V3, V4 and V5. Among these, V0 and V5 are supplied by a power supply source or an emitter follower employing a transistor. Furthermore, when a display capacity of the liquid crystal is comparatively small, as shown in an example of FIG. 10A, they are directly supplied to driver ICs from divided resistors. When the display capacity thereof is comparatively large, as shown in an example of FIG. 10B, they are inputted to predetermined terminals of the respective driver ICs whereby impedances thereof are lowered by inserting voltage followers employing operational amplifiers after the divided resistors.
The driver IC is a driving means having a function whereby a row electrode waveform composed of a selecting voltage and a non-selecting voltage, is applied on a row electrode of a matrix-type display device, and a column electrode waveform composed of an ON-voltage and an OFF-voltage, is controlled and applied on a column electrode. In FIGS. 10A and 10B, V.sub.adj designates a control voltage which is supplied for controlling the liquid crystal display panel to be provided with a brightness which is easy to see.
However, even in a circuit inserted with the voltage followers after the divided resistors as shown in FIG. 10B, the voltages V1 through V4 are not stable since they are superposed with various noises. Accordingly, there is a variation among root mean square voltages applied on the respective display dots, and a nonuniformity of display is caused.
It is an object of the present invention to provide an image display device having a uniform, with a small nonuniformity of display and easy-to-see image face, wherein a voltage distortion in a spike-like form is reduced by an effective feedback circuit.